1. Field of the Invention
The present invention relates to the adjustment of oil-less positive displacement rotary screw compressors. More particularly, the present invention provides a new method and apparatus for accurately adjusting the flank clearances between the rotors of oil-less positive displacement rotary screw compressors.
2. Background of the Related Art
Rotary screw compressors are used in numerous industries to provide a supply of compressed air for supporting applications such as automatic machines, tools, material handling devices, and food processing equipment. In comparison to the predominate reciprocating-piston type compressor, rotary screw compressors operate more efficiently and at a lower compressor specific power, providing small capacities at high pressures. Other advantages include reduced space requirements and lower vibration levels. Two types of rotary screw compressors are oil-injected and oil-less.
The oil-injected type rotary screw compressor includes a casing with two intersecting bores having parallel axes, an inlet port adjacent one endwall, and a compressed air outlet port adjacent another endwall. Disposed within the bores are a pair of meshing rotors--each rotor having helical lands and intervening grooves with a wrap angle of less than 360.degree.. The leading and trailing faces of each land form leading and trailing flanks. Minimal clearances are maintained between the rotors and the end walls and bores of the casing. One rotor is a male rotor type, i.e., a rotor having at least the major portions of its lands and grooves disposed outside the pitch circle of the rotor. The other rotor is a female rotor type, i.e., a rotor having at least the major portions of its lands and grooves disposed inside the pitch circle of the rotor. The lands of one rotor follow the envelopes developed by the grooves of the other rotor to form a continuous sealing line there between. Chambers are formed between the sealing line, land tops, casing end walls and bores. One of the rotors is driven by a motor while the other rotor is driven by the first.
In operation, a gaseous fluid is displaced and compressed within the chambers from the inlet port to the outlet port of the compressor. Three phases make up this process: a filling phase, a compression phase, and a discharge phase. During the filling phase each compression chamber communicates with the air inlet port, during the compression phase the chamber undergoes a continued reduction in volume, and during the discharge phase the chamber communicates with the compressed air outlet port.
Because the flanks of the rotors of the above described screw compressor are in meshing contact with one another, oil must be injected into the compressor to prevent excessive contact wear which would ultimately lead to premature failure of the compressor. Oil-injected systems, however, have several drawbacks. Although the contact wear is substantially reduced due to the lubricating film, it is not eliminated. Also, the injected oil necessarily passes into the air system that is being supplied by the compressor. Oil separation units can be included in the compressed air line to substantially reduce the quantity of oil that enters the air line, however the separation units never completely eliminate it. In certain applications, such as food processing equipment and hospital air systems, even traces of oil bypass is impermissible.
To eliminate the problems associated with oil-injected systems, oil-less type rotary screw compressors have been developed. The essential difference between oil-injected and oil-less air compressors is that the male and female rotors of oil-less systems are timed so they do not come into contact with each other during operation. In other words, total backlash between the rotors is proportioned, not necessarily equally, between the male rotor leading flank and female rotor trailing flank, and the male rotor trailing flank and female rotor leading flank. Rotor timing is typically provided by either helical or spur gears having pitch circles matching the pitch circles of their respective rotors.
During assembly, overhaul, and periodically during the maintenance of oil-less compressors, the rotors must be properly adjusted and their gears set to ensure the proper clearance between the rotor flanks. Improper clearance settings can cause poor compressor performance, excessive operating noise, or even destructive failure of the compressor. The amount of rotor clearance depends on the compressor design and, therefore, must be determined by the manufacturer of each compressor. Rotor clearance adjustments must be made after the compressor has been assembled but prior to operation.
Presently, a technician sets rotor clearance by inserting a feeler gauge between the flanks of a set of rotor lands and adjusting the rotors to the predetermined clearance. Access to the rotors is gained by reaching through either the inlet port or the outlet port. The timing gears are then adjusted and secured to retain the gauged setting.
There are numerous disadvantages associated with the above method of rotor adjustment. The inlet and outlet ports are small, therefore making it very difficult to accurately insert the feeler gauges between the rotor lands. Also, even if ready access could be had, it is difficult to accurately locate the feeler gauge at the exact point of flank contact between the rotors. Additionally, once the feeler gauges are positioned between the rotor lands and the rotors are rotated to the gauged setting, it is difficult to maintain the rotor relationship while adjusting and securing the timing gears in position. This later step usually requires a second technician's assistance. Because of the above disadvantages, rotor adjustments must be made by experienced technicians and, even then, takes a great deal of time and care to properly accomplish.
Clearly there is a need in the art for a method and an apparatus to accurately and easily adjust and set the flank clearance between the rotors of a rotary screw compressor. Preferably, the method should be able to be performed and the apparatus utilized externally to the compressor casing. In addition, the method and apparatus should permit the user to clamp the gauged rotors firmly in place while fastening the timing gears. Finally, successful use of the method and apparatus should not be dependent on the experience of the technician charged with adjusting and setting the rotor flank clearance.